Conventionally, in order to obtain a large adhesion strength when using a copolymer latex as a binder, there have been studied a method in which a molecular weight, degree of crosslinking, glass transition temperature and the like of a copolymer in the copolymer latex are optimized, and, in a copolymer latex used as a binder of a coating composition containing filler or the like, a method in which an adhesion strength is enhanced by downsizing the copolymer particles to thereby increase an effective adhesion area between the filler and copolymer particles.
For example, as a binder of a composition for paper coating used for production of coated papers, there has been known a copolymer latex having a small particle size is used for suppressing irregular flow of a coating liquid under a high shearing velocity (PTL 1 to PTL 6), and when using the copolymer latex having a small particle size, it is expected that the resulting adhesion strength is improved. However, since the copolymer latex having a small particle size has an extremely high viscosity, it is very difficult to be handled at a practical concentration unless a large amount of a viscosity reducing agent is used.
In contrast, a copolymer latex using a large amount of a viscosity reducing agent made it difficult to obtain practical performances as a binder, because of deteriorating of water resistance and the like. Therefore, although there are described in documents, the particle size of the copolymer latex actually in practical use is not made to be as small as having sufficient adhesion strength. A latex having sufficiently smaller particle size has not yet been practically used because of lowering of handling properties and other properties, or has had insufficient compatibility with other performances required as a binder.
In the technique disclosed in PTL 1, since it is impossible to improve both printing strength and blister resistance at the same time only by making a particle size of a copolymer in latex small, attention is focused on a gel content of the latex. However, the latex contains a large amount of surfactant used in its production, and thus the resulting coating layer by using the latex is considered to have insufficient wet pick strength.
PTL 2 discloses the use of a specific reactive emulsifier for producing stably a latex having a small particle size of 50 nm or less which is used for high speed coating, without using a large amount of an emulsifier. However in accordance with this production method, the resulting latex has high viscosity, and thus coating workability is not yet sufficient.
In PTL 3, there is disclosed a latex having a number average particle size of 80 nm or less. The technique in the Literature focuses attention on a contact angle of the latex in order to improve printing properties, and the Literature discloses use of a particular anionic surfactant for producing a latex having the specific contact angle.
In PTL 4, it is disclosed that a latex having an average particle size of 30 to 50 nm is produced by using a polymerization inhibitor in the pre-stage of two-stage polymerization, and the obtained latex is considered to have a high viscosity in view of monomer composition.
In PTL 5, there is disclosed a method for producing a latex having a number average particle size of 100 nm or less by continuous addition polymerization, and in this method, stability of the resulting latex is considered to have a certain problem because the amount of an acid monomer to be used is small.
In PTL 6, there is disclosed a process for producing a latex having a number average particle size of 60 nm or less by performing polymerization through the use of 2,4-dinitrochlorobenzen and under polymerization conditions in which the maximum value of an instantaneous polymerization rate is 10 to 45% by mass/hour, and in this method, the resulting latex is considered to have poor water resistance because the amount of an emulsifier to be used is large.